Drug-resistant Plasmodium falciparum is a major threat to global public health and new strategies are needed to deter spread of resistance to available and forthcoming antimalarial drugs. As in other South American settings, there is a high level of chloroquine resistance on the Pacific coast of Colombia. However, resistance to the combination antifolate drug sulfadoxine-pyrimethamine (SP) has remained low in Colombia in contrast to high rates of SP failure elsewhere in the region. Therefore research efforts are urgently needed to maximize the longevity of SP as an effective treatment. Point mutations in parasite dihydrofolate reductase (DHFR) and dihydropteroate synthase (DRPS) confer in vitro resistance to pyrimethamine and sulfadoxine, respectively. These resistance-conferring mutations occur in a stepwise, sequential fashion, with higher levels of in vitro resistance occurring in the presence of multiple mutations. However, factors contributing to host-vector transmission of resistance are less well understood and little is known about the capacity for transmission of resistant parasites. Recently we reported that parasite mutations conferring low-level resistance to SP, even though insufficient to cause treatment failure, may contribute to the potential for the transmission of Plasmodium falciparum and the spread of resistance. We found that DHFR mutations in asexual parasites were associated with longer parasite clearance time (PCT) and the presence of gametocytes (the sexual form of the parasite that is transmitted by the mosquito) after SP treatment. This finding suggests that even before clinical SP resistance is apparent, drug treatment may be responsible for selecting resistant parasites and contributing to the spread of resistance. Confirmation of this preliminary result by the proposed studies will support routine inclusion of gametocytocidal agents in antimalarial regimens. This study seeks to contribute to understanding the molecular basis of spread of drug resistance in a setting with low level of SP treatment failure. The overall aim of the project is to assess the association between the occurrence of mutations in P. falciparum DHFR and DHPS genes and transmission capacity of parasites to Anopheles mosquitoes. We plan to conduct a cohort study at the Pacific coast of Colombia where we have carried out malaria research for the past 10 years. Patients with uncomplicated falciparum malaria will receive and standard SP regimen and will be closely followed for 28 days. This study will increase our understanding of how drug-resistant malaria is spread and is expected to provide a sound basis for the future clinical evaluation of antimalarial drug combinations designed to prevent transmission of drug-resistant malaria parasites.